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As soon as a pool reached pm.max_children, Nginx starts timing out when trying to send new requests to PHP-FPM. "max listen queue" is always 0 in the php-status page.

  • PHP-FPM 5.5.16
  • Nginx 1.6.1

Here is the example for a php-fpm pool:

[example] catch_workers_output = no ; Configure listener listen = /var/run/php-fpm/example.sock listen.backlog = 65535 listen.owner = nginx listen.group = nginx ; Unix user/group of processes user = nginx group = nginx ; Choose how the process manager will control the number of child processes. pm = ondemand pm.max_children = 10 pm.max_requests = 200 pm.process_idle_timeout = 30s pm.status_path = /status ; Pass environment variables env[HOSTNAME] = $HOSTNAME env[PATH] = /usr/local/bin:/usr/bin:/bin env[TMP] = /tmp env[TMPDIR] = /tmp env[TEMP] = /tmp ; Host specific php ini settings here php_admin_flag[log_errors] = on php_admin_value[open_basedir] = /tmp:/var/www/apc:/var/www/wordpress/example php_admin_value[error_log] = /var/log/php-fpm/example.log
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  • did you ever find out what's wrong? Commented Jun 10, 2016 at 10:01

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As this question still shows up in the unanswered questions, I'll try with a outdated answer. Showing an error is the intended behavior of the setting "pm.max_children", according to the PHP manual:

The number of child processes ... to be created when pm is set to dynamic.

This option sets the limit on the number of simultaneous requests that will be served.

However, each request should be handled quite quickly, so the process becomes free for the next request. If not, nginx will probably report a "502 Bad Gateway" as soon as it cannot handle more requests.

Double-check the value set in the php-fpm configuration for listen.backlog. This defines the queue length (reference):

The backlog argument defines the maximum length to which the queue of pending connections

However, this value is limited by the underlying system. See:

sysctl net.core.somaxconn

As far as I know, there's no way to queue a request to the upstream (php-fpm), if this throws an error. Yet, you may tell nginx to switch to another process, if the error occurs. This may trigger a client-side reload, for example.

If it's not the listen-backlog/net.core.somaxconn setting, then the actual question, however, is why requests block the php-fpm process for so long.

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